Process for complex shape formation using flexible graphite sheets

ABSTRACT

Processes are provided for formation of complex shapes by embossing of a sheet of flexible graphite material. In one approach, a sheet of material is provided with a variable resin concentration across its width, and the position of the variable resin concentration is correlated with the position of embossing features which will result in thinner areas in the embossed articles. In a second approach, recesses are provided in the embossing rollers to accommodate material flow during embossing. These recesses result in protrusions formed on the articles, which protrusions must then be removed in a machining operation.

TECHNICAL FIELD

[0001] The present invention relates generally to processes and systemsfor manufacturing articles from sheets of flexible graphite material,and more particularly to systems for forming complex shapes wherein thesystems are designed to control the flow of the graphite material andresin in the sheets during an embossing process.

BACKGROUND OF THE INVENTION

[0002] An ion exchange membrane fuel cell, more specifically a protonexchange membrane (PEM) fuel cell, produces electricity through thechemical reaction of hydrogen and oxygen in the air. Within the fuelcell, electrodes denoted as anode and cathode surround a polymerelectrolyte to form what is generally referred to as a membraneelectrode assembly, or MEA. Oftentimes, the electrodes also function asthe gas diffusion layer (or GDL) of the fuel cell. A catalyst materialstimulates hydrogen molecules to split into hydrogen atoms and then, atthe membrane, the atoms each split into a proton and an electron. Theelectrons are utilized as electrical energy. The protons migrate throughthe electrolyte and combine with oxygen and electrons to form water.

[0003] A PEM fuel cell is advantageously formed of a membrane electrodeassembly sandwiched between two graphite flow field plates.Conventionally, the membrane electrode assembly consists ofrandom-oriented carbon fiber paper electrodes (anode and cathode) with athin layer of a catalyst material, particularly platinum or a platinumgroup metal coated on isotropic carbon particles, such as lamp black,bonded to either side of a proton exchange membrane disposed between theelectrodes. In operation, hydrogen flows through channels in one of theflow field plates to the anode, where the catalyst promotes itsseparation into hydrogen atoms and thereafter into protons that passthrough the membrane and electrons that flow through an external load.Air flows through the channels in the other flow field plate to thecathode, where the oxygen in the air is separated into oxygen atoms,which joins with the protons through the proton exchange membrane andthe electrons through the circuit, and combine to form water. Since themembrane is an insulator, the electrons travel through an externalcircuit in which the electricity is utilized, and join with protons atthe cathode. An air stream on the cathode side is one mechanism by whichthe water formed by combination of the hydrogen and oxygen is removed.Combinations of such fuel cells are used in a fuel cell stack to providethe desired voltage.

[0004] One limiting factor to the use of graphite materials, especiallyflexible graphite materials, as components for PEM fuel cells is thedefinition of a pattern embossed on the material, which, if notsufficient, can interfere with operation of the fuel cell, by permittingleaking of fluids, not permitting sufficient fluid flow through the fuelcell, or changing load and/or current paths through the cell.

[0005] Graphite's are made up of layer planes of hexagonal arrays ornetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another. Thesubstantially flat, parallel equidistant sheets or layers of carbonatoms, usually referred to as graphene layers or basal planes, arelinked or bonded together and groups thereof are arranged incrystallites. Highly ordered graphites consist of crystallites ofconsiderable size: the crystallites being highly aligned or orientedwith respect to each other and having well ordered carbon layers. Inother words, highly ordered graphites have a high degree of preferredcrystallite orientation. It should be noted that graphites by definitionpossess anisotropic structures and thus exhibit or possess manyproperties that are highly directional e.g. thermal and electricalconductivity and fluid diffusion.

[0006] Briefly, graphites may be characterized as laminated structuresof carbon, that is, structures consisting of superposed layers orlaminae of carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

[0007] As noted above, the bonding forces holding the parallel layers ofcarbon atoms together are only weak van der Waals forces. Naturalgraphites can be chemically treated so that the spacing between thesuperposed carbon layers or laminae can be appreciably opened up so asto provide a marked expansion in the direction perpendicular to thelayers, that is, in the “c” direction, and thus form an expanded orintumesced graphite structure in which the laminar character of thecarbon layers is substantially retained.

[0008] Graphite flake which has been chemically or thermally expandedand more particularly expanded so as to have a final thickness or “c”direction dimension which is as much as about 80 or more times theoriginal “c” direction dimension can be formed without the use of abinder into cohesive or integrated sheets of expanded graphite, e.g.webs, papers, strips, tapes, or the like (typically referred to as“flexible graphite”). The formation of graphite particles which havebeen expanded to have a final thickness or “c” dimension which is asmuch as about 80 times or more the original “c” direction dimension intointegrated flexible sheets by compression, without the use of anybinding material, is believed to be possible due to the mechanicalinterlocking, or cohesion, which is achieved between the voluminouslyexpanded graphite particles.

[0009] In addition to flexibility, the sheet material, as noted above,has also been found to possess a high degree of anisotropy with respectto thermal and electrical conductivity and fluid diffusion, comparableto the natural graphite starting material due to orientation of theexpanded graphite particles substantially parallel to the opposed facesof the sheet resulting from very high compression, e.g. roll pressing.Sheet material thus produced has excellent flexibility, good strengthand a very high degree of orientation.

[0010] Briefly, the process of producing flexible, binderlessanisotropic graphite sheet material, e.g. web, paper, strip, tape, foil,mat, or the like, comprises compressing or compacting under apredetermined load and in the absence of a binder, expanded graphiteparticles which have a “c” direction dimension which is as much as about80 or more times that of the original particles so as to form asubstantially flat, flexible, integrated graphite sheet. The expandedgraphite particles that generally are worm-like or vermiform inappearance, once compressed, will maintain the compression set andalignment with the opposed major surfaces of the sheet. The density andthickness of the sheet material can be varied by controlling the degreeof compression. The density of the sheet material is typically withinthe range of from about 0.04 g/cc to about 1.4 g/cc. The flexiblegraphite sheet material exhibits an appreciable degree of anisotropy dueto the alignment of graphite particles parallel to the major opposed,parallel surfaces of the sheet, with the degree of anisotropy increasingupon roll pressing of the sheet material to increased density. In rollpressed anisotropic sheet material, the thickness, i.e. the directionperpendicular to the opposed, parallel sheet surfaces comprises the “c”direction and the directions ranging along the length and width, i.e.along or parallel to the opposed, major surfaces comprises the “a”directions and the thermal, electrical and fluid diffusion properties ofthe sheet are very different, by orders of magnitude typically, for the“c” and “a” directions.

[0011] This considerable difference in properties, i.e. anisotropy,which is directionally dependent, can be disadvantageous in someapplications. For example, in gasket applications where flexiblegraphite sheet is used as the gasket material and in use is held tightlybetween metal surfaces, the diffusion of fluid, e.g. gases or liquids,occurs more readily parallel to and between the major surfaces of theflexible graphite sheet. It would, in most instances, provide forgreater gasket performance, if the resistance to fluid flow parallel tothe major surfaces of the graphite sheet (“a” direction) were increased,even at the expense of reduced resistance to fluid diffusion flowtransverse to the major faces of the graphite sheet (“c” direction).With respect to electrical properties, the resistivity of anisotropicflexible graphite sheet is high in the direction transverse to the majorsurfaces (“c” direction) of the flexible graphite sheet, andsubstantially less in the direction parallel to the major faces of theflexible graphite sheet (“a” direction). In applications such aselectrodes for fuel cells, it would be of advantage if the electricalresistance transverse to the major surfaces of the flexible graphitesheet (“c” direction) were decreased, even at the expense of an increasein electrical resistivity in the direction parallel to the major facesof the flexible graphite sheet (“a” direction).

[0012] With respect to thermal properties, the thermal conductivity of aflexible graphite sheet in a direction parallel to the major surfaces ofthe flexible graphite sheet is relatively high, while it is relativelylow in the “c” direction transverse to the major surfaces.

[0013] Flexible graphite sheet can also be provided with channels, whichare preferably smooth-sided, and which pass between the parallel,opposed surfaces of the flexible graphite sheet and are separated bywalls of compressed expanded graphite. When such a flexible graphitesheet functions as an electrode in an electrochemical fuel cell, it isplaced so as to abut the ion exchange membrane, so that the “tops” ofthe walls of the flexible graphite sheet abut the ion exchange membrane.

[0014] The materials used to form components of fuel cells such aselectrodes and flow field plates can be complex shapes which require asubstantial amount of material movement when forming these shapes byembossing a resin impregnated flexible graphite sheet.

[0015] There is a continuing need for improved processes for themanufacture of such complex parts, and particularly for processeswherein the required movement of material is minimized so as to minimizetearing, warpage and the like of the flexible graphite sheet and thearticles formed therefrom. The present invention provides such improvedprocesses, which are particularly useful in the manufacture of materialsthat can be formed into components of electrochemical fuel cells, suchas electrodes and flow field plates.

SUMMARY OF THE INVENTION

[0016] The present invention provides two approaches to solving thisproblem. In the first approach, a variable resin concentration isprovided across the width of a sheet of material which is to beembossed, and the areas of lesser resin concentration are thencorrelated with areas of an embossing pattern having a lessercross-section (that is a narrower gap between embossing rollers) so thatthe amount of resin which must be moved by the embossing pattern inthese areas of lesser cross-section is reduced as compared to what wouldbe required if embossing a sheet having uniform resin content across itswidth. The second approach to this problem utilizes a sheet of graphitematerial having substantial uniform thickness and uniform resinconcentration, and modifies the pair of embossing rollers so as to allowspace for movement of the graphite material to form a protrusion whichcan subsequently be removed.

[0017] In the first approach, a process is provided for forming articlesfrom graphite material, which process includes the steps of:

[0018] (a) providing a sheet of variably impregnated flexible graphitematerial having a variable resin concentration across a width of thesheet; and

[0019] (b) passing the variably impregnated sheet past an embossingroller having an embossing pattern of variable cross-section across thewidth of the sheet, the embossing pattern having areas of lesser sheetcross-section which correspond to areas of lesser resin content of thevariably impregnated sheet, so that an amount of resin which must bemoved lengthwise along the sheet during the embossing process is reducedas compared to a sheet having uniform resin content across its width.

[0020] The methods of the second approach can be described as methods ofmanufacturing articles having a variable thickness from a sheet offlexible graphite material of uniform thickness, the methods includingthe steps of:

[0021] (a) Providing a sheet of impregnated flexible graphite materialhaving a substantially uniform thickness across a width of the sheet;

[0022] (b) Embossing the sheet between an embossing element and alanding element having embossing and landing surfaces, respectively, atleast one of which surfaces includes a first recess for forming aprotrusion on the embossed sheet; and

[0023] (c) Removing the protrusion thereby providing an article having avariable thickness along a dimension of the article corresponding to thewidth of the sheet.

[0024] Accordingly, it is an object of the present invention to providesystems and processes for the manufacture of complex shapes from sheetsof flexible graphite material.

[0025] Another object of the present invention is the provision of aprocess for providing a sheet of variably impregnated flexible graphitematerial having a variable resin concentration across a width of thesheet.

[0026] Another object of the present invention is the provisions ofsystems having embossing patterns of variable cross-section which arecorrelated with the presence of variable resin concentrations in a sheetof flexible graphite material to be embossed.

[0027] And another object of the present invention is the provision ofsystems and processes for manufacturing articles from sheets of flexiblegraphite material of uniform thickness, wherein provision is made forthrough-plane movement of graphite material thus forming protrusionswhich will later be removed, as contrasted to requiring lateral movementof material to achieve the embossing process.

[0028] Another object of the present invention is the provision ofprocesses for embossing articles from flexible graphite material withoutrupturing or tearing the material during the embossing process.

[0029] Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the following disclosure when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a perspective schematic view of an embossing processwherein a pair of precalendering rollers creates a high density strip ina sheet of flexible graphite material, which results in a lower resinconcentration along that strip after impregnation and drying. The stripis correlated in position to a high flow area of the embossing patternof the embossing rollers, so that there is less material which must bedisplaced by the high flow area of the embossing rollers.

[0031]FIG. 2 is a schematic elevation side view of a precalenderingroller having a complex outer surface so that several different resinweight percentage areas are created across the width of the sheet offlexible graphite material.

[0032]FIG. 3 is a schematic illustration of a portion of the processline wherein a finishing station is provided downstream of the embossingrollers. The finishing station provides a means for grinding or slicingoff extruded excess material from the embossed article.

[0033]FIG. 4 is a schematic plan view of a complex article such as aflow field plate for a fuel cell.

[0034]FIG. 5 is a schematic plan view similar to FIG. 4 showing thelocation of gates in the backup roller adjacent the periphery of thearticle thus promoting in-plane resin and graphite flow during theembossing operation.

[0035]FIGS. 6 and 7 are plan and side views, respectively, of an articleshowing the position of gates on the backside of the article, whichgates are formed in the backup roller, thus promoting through-planeresin and graphite flow to accommodate the required materialdisplacement for formation of the article.

[0036]FIG. 8 is a schematic side elevation view of a portion of themanufacturing process wherein the embossing rollers includecomplementary positive and negative patterns to create a constantcross-sectional area across the article being formed, followed by afinishing station for removal of the excess material extruded to thebackside of the article.

[0037]FIG. 9 is a plan view of the article formed in the process of FIG.8.

[0038]FIG. 10 is a side view of the article of FIG. 9, showing a fin orridge which has been formed on the backside by the embossing process andwhich must be removed at the finishing station of FIG. 8.

[0039]FIG. 11 is a perspective view of the embossed but not yet finallyfinished article of FIGS. 9 and 10.

[0040]FIGS. 12, 13 and 14 are cross-sectional views taken along lines12-12, 13-13 and 14-14 of FIG. 11, respectively, to show the variouscross-sectional shapes resulting from the embossing step of FIG. 8,prior to the finishing/material removal step of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Graphite is a crystalline form of carbon comprising atomscovalently bonded in flat layered planes with weaker bonds between theplanes. By treating particles of graphite, such as natural graphiteflake, with an intercalant of, e.g. a solution of sulfuric and nitricacid, the crystal structure of the graphite reacts to form a compound ofgraphite and the intercalant. The treated particles of graphite arehereafter referred to as “particles of intercalated graphite.” Uponexposure to high temperature, the intercalant within the graphitedecomposes and volatilizes, causing the particles of intercalatedgraphite to expand in dimension as much as about 80 or more times itsoriginal volume in an accordion-like fashion in the “c” direction, i.e.in the direction perpendicular to the crystalline planes of thegraphite. The exfoliated graphite particles are vermiform in appearance,and are therefore commonly referred to as worms. The worms may becompressed together into flexible sheets that, unlike the originalgraphite flakes, can be formed and cut into various shapes and providedwith small transverse openings by deforming mechanical impact.

[0042] Graphite starting materials suitable for use in the presentinvention include highly graphitic carbonaceous materials capable ofintercalating organic and inorganic acids as well as halogens and thenexpanding when exposed to heat. These highly graphitic carbonaceousmaterials most preferably have a degree of graphitization of about 1.0.As used in this disclosure, the term “degree of graphitization” refersto the value g according to the formula:$g = \frac{3.45 - {d(002)}}{0.095}$

[0043] where d(002) is the spacing between the graphitic layers of thecarbons in the crystal structure measured in Angstrom units. The spacingd between graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as carbons prepared bychemical vapor deposition and the like. Natural graphite is mostpreferred.

[0044] The graphite starting materials used in the present invention maycontain non-carbon components so long as the crystal structure of thestarting materials maintains the required degree of graphitization andthey are capable of exfoliation. Generally, any carbon-containingmaterial, the crystal structure of which possesses the required degreeof graphitization and which can be exfoliated, is suitable for use withthe present invention. Such graphite preferably has an ash content ofless than about six weight percent. More preferably, the graphiteemployed for the present invention will have a purity of at least about98%. In the most preferred embodiment, the graphite employed will have apurity of at least about 99%.

[0045] A common method for manufacturing graphite sheet is described byShane et al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

[0046] In a preferred embodiment, the intercalating agent is a solutionof a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, andan oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

[0047] The quantity of intercalation solution may range from about 20 toabout 150 pph and more typically about 50 to about 120 pph. After theflakes are intercalated, any excess solution is drained from the flakesand the flakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 50pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

[0048] The particles of graphite flake treated with intercalationsolution can optionally be contacted, e.g. by blending, with a reducingorganic agent selected from alcohols, sugars, aldehydes and esters whichare reactive with the surface film of oxidizing intercalating solutionat temperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

[0049] The use of an expansion aid applied prior to, during orimmediately after intercalation can also provide improvements. Amongthese improvements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

[0050] Representative examples of saturated aliphatic carboxylic acidsare acids such as those of the formula H(CH₂)_(n)COOH wherein n is anumber of from 0 to about 5, including formic, acetic, propionic,butyric, pentanoic, hexanoic, and the like. In place of the carboxylicacids, the anhydrides or reactive carboxylic acid derivatives such asalkyl esters can also be employed. Representative of alkyl esters aremethyl formate and ethyl formate. Sulfuric acid, nitric acid and otherknown aqueous intercalants have the ability to decompose formic acid,ultimately to water and carbon dioxide. Because of this, formic acid andother sensitive expansion aids are advantageously contacted with thegraphite flake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

[0051] The intercalation solution will be aqueous and will preferablycontain an amount of expansion aid of from about 1 to 10%, the amountbeing effective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

[0052] After intercalating the graphite flake, and following theblending of the intercalant coated intercalated graphite flake with theorganic reducing agent, the blend is exposed to temperatures in therange of 25° to 125° C. to promote reaction of the reducing agent andintercalant coating. The heating period is up to about 20 hours, withshorter heating periods, e.g., at least about 10 minutes, for highertemperatures in the above-noted range. Times of one half hour or less,e.g., on the order of 10 to 25 minutes, can be employed at the highertemperatures.

[0053] The thus treated particles of graphite are sometimes referred toas “particles of intercalated graphite.” Upon exposure to hightemperature, e.g. temperatures of at least about 160° C. and especiallyabout 700° C. to 1000° C. and higher, the particles of intercalatedgraphite expand as much as about 80 to 1000 or more times their originalvolume in an accordion-like fashion in the c-direction, i.e. in thedirection perpendicular to the crystalline planes of the constituentgraphite particles. The expanded, i.e. exfoliated, graphite particlesare vermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into flexible sheets that,unlike the original graphite flakes, can be formed and cut into variousshapes and provided with small transverse openings by deformingmechanical impact as hereinafter described.

[0054] Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll-pressing, to athickness of about 0.075 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cc). From about 1.5-30% byweight of ceramic additives can be blended with the intercalatedgraphite flakes as described in U.S. Pat. No. 5,902,762 (which isincorporated herein by reference) to provide enhanced resin impregnationin the final flexible graphite product. The additives include ceramicfiber particles having a length of about 0.15 to 1.5 millimeters. Thewidth of the particles is suitably from about 0.04 to 0.004 mm. Theceramic fiber particles are non-reactive and non-adhering to graphiteand are stable at temperatures up to about 1100° C., preferably about1400° C. or higher. Suitable ceramic fiber particles are formed ofmacerated quartz glass fibers, carbon and graphite fibers, zirconia,boron nitride, silicon carbide and magnesia fibers, naturally occurringmineral fibers such as calcium metasilicate fibers, calcium aluminumsilicate fibers, aluminum oxide fibers and the like.

[0055] The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, eliminatesthrough-plane permeability while increasing handling strength, i.e.stiffness, of the flexible graphite sheet as well as “fixing” themorphology of the sheet. Suitable resin content is preferably at leastabout 5% by weight, more preferably about 10 to 45% by weight, andsuitably up to about 60% by weight. Resins found especially useful inthe practice of the present invention include acrylic-, epoxy- andphenolic-based resin systems, or mixtures thereof. Suitable epoxy resinsystems include those based on diglycidyl ether of bisphenol A (DGEBA)and other multifunctional resin systems; phenolic resins that can beemployed include resole and novolac phenolics.

[0056] In a typical resin impregnation step, the flexible graphite sheetis passed through a vessel and impregnated with the resin system from,e.g. spray nozzles, the resin system advantageously being “pulledthrough the mat” by means of a vacuum chamber. The resin is thereafterpreferably dried, reducing the tack of the resin and theresin-impregnated sheet, which has a starting density of about 0.1 toabout 1.1 g/cc, is thereafter processed to change the void condition ofthe sheet. By void condition is meant the percentage of the sheetrepresented by voids, which are generally found in the form of entrappedair. Generally, this is accomplished by the application of pressure tothe sheet (which also has the effect of densifying the sheet) so as toreduce the level of voids in the sheet, for instance in a calender millor platen press. Advantageously, the flexible graphite sheet isdensified to a density of at least about 1.3 g/cc (although the presenceof resin in the system can be used to reduce the voids without requiringdensification to so high a level).

[0057] The void condition can be used advantageously to control andadjust the morphology and functional characteristics of the finalembossed article. For instance, thermal and electrical conductivity,permeation rate and leaching characteristics can be effected andpotentially controlled by controlling the void condition (and, usually,the density) of the sheet prior to embossing. Thus, if a set of desiredcharacteristics of the final embossed article is recognized prior tomanipulation of the void condition, the void condition can be tailoredto achieve those characteristics, to the extent possible.

[0058] Advantageously, especially when the final embossed article isintended for use as a component in an electrochemical fuel cell, theresin-impregnated flexible graphite sheet is manipulated so as to berelatively void-free, to optimize electrical and thermal conductivities.Generally, this is accomplished by achieving a density of at least about1.4 g/cc, more preferably at least about 1.6 g/cc (depending on resincontent), indicating a relatively void-free condition.

[0059] The calendered flexible graphite sheet is then passed through anembossing apparatus as described herein below, and thereafter heated inan oven to cure the resin. Depending on the nature of the resin systememployed, and especially the solvent type and level employed (which isadvantageously tailored to the specific resin system, as would befamiliar to the skilled artisan), a vaporization drying step may beincluded prior to the embossing step. In this drying step, the resinimpregnated flexible graphite sheet is exposed to heat to vaporize andthereby remove some or all of the solvent, without effecting cure of theresin system. In this way, blistering during the curing step, which canbe caused by vaporization of solvent trapped within the sheet by thedensification of the sheet during surface shaping, is avoided. Thedegree and time of heating will vary with the nature and amount ofsolvent, and is preferably at a temperature of at least about 65° C. andmore preferably from about 80° C. to about 95° C. for about 3 to about20 minutes for this purpose.

[0060] One embodiment of an apparatus for continuously formingresin-impregnated and calendared flexible graphite sheet is shown inInternational Publication No. WO 00/64808 the disclosure of which isincorporated herein by reference.

[0061] Referring now to the drawings, and particularly to FIG. 1, asystem or processing line for forming articles from an elongated sheet12 of flexible graphite material is shown and generally designated bythe numeral 10.

[0062] In the system 10, the sheet 12 of flexible graphite material isshown in a continuous process. The sheet 12 begins at the left hand endof the system 10 as a sheet of mat material 14. The mat material 14moves through a pair of precalendering rollers 16 and 18 and exits theprecalendering operation as a precalendered mat 20 having a high densitystrip 22 defined therein. The high density strip 22 is formed by aprotruding ridge 23 on the roller 16.

[0063] The precalender strip 20 then moves through a station 24 at whichthe mat is impregnated, dried and calendered to form a sheet 26 of resinimpregnated flexible graphite material having a strip 28 of relativelylower resin density. The sheet 26 then moves through an embossingstation where it is passed between embossing rollers 30 and 32. Theembossing roller 30 has an embossing pattern defined thereon, whichpattern includes embossing features 33 and 34. Feature 34 of theembossing pattern is higher than surrounding areas of the pattern andthus the resulting thickness of the finished article will be thinner.This feature 34 which creates a high material flow corresponds to theposition of the strip 28 of lower resin content, so that there is lessresin material which must be displaced by the feature 34.

[0064] The embossing rollers 30 result in embossed articles 36 a, 36 b,36 c, etc. being formed in a continuous series in the sheet 26. Thearticles 36 will subsequently be separated from each other.

[0065] The process performed by the system 10 can generally be describedas a process of forming articles 36 from a sheet 12 of flexible graphitematerial.

[0066] The process includes a first step of providing a sheet 26 ofvariably impregnated flexible graphite material having a variable resinconcentration across the width 38 of the sheet. As previously noted, thestrip 28 will have a lower resin concentration. This is because thestrip 22 in the precalendered mat 20 has been compressed to a higherdensity than the remainder of the precalendered mat 20 and thus willabsorb less resin when a resin and acetone solvent carrier mixture issprayed upon the precalendered mat 20 in the processing station 24.Since less resin will be absorbed in the high density strip 22, therewill be a lesser resin concentration in the strip 28 of the sheet 26.

[0067] The second step in the process, which is performed by embossingrollers 30 and 32, can be described as passing the variably impregnatedsheet 26 past an embossing roller 30 having an embossing pattern 33, 34of variable cross- section areas across the width 38 of the sheet 26.

[0068] The embossing pattern has a feature such as 34 which creates anarea of lesser cross-section in the finished article, which correspondsto the areas 28 of lesser resin content of the variably impregnatedsheet 26, so that an amount of resin which must be moved lengthwisealong the sheet within the plane of the sheet 26 during the embossingprocess is reduced as compared to the amount of material which wouldhave to be moved in a sheet of uniform resin content across its width.

[0069] The steps performed by the precalendering rollers 16 and 18 andby the processing station 24 can be described as precalendering thesheet 12 when the sheet is in the form of an unimpregnated mat 14, sothat the mat 14 has a variable density across the width 38, and thenimpregnating the variable density mat 20 with resin at processingstation 24, so that an area 22 of higher density mat becomes an area 28of lower weight percentage resin concentration across the width 38 ofthe sheet 26.

[0070] In the processing station 24, the entire width 38 of the sheet 20of precalendered mat will be sprayed with the same constant resinsolution, so that the variable resin concentration of strip 28 is afunction solely of the variable density of the strip 22 of theprecalendered mat 20.

[0071] As shown in FIG. 2, the precalendering roller 16 can have an area40 of very complex cross-section which will result in a complexvariation in density across the width of the precalendered sheet 20 thusresulting in a complex variation of resin concentration across the widthof the sheet 26. This can be described as creating at least three areaseach having a different mat density across the width of theprecalendered mat 20.

[0072] Turning now to FIGS. 3-14, another approach is shown for themanufacture of complex shaped articles through embossing of flexiblegraphite sheets.

[0073] The approaches of the process illustrated in FIGS. 3-7 and thatof FIGS. 8-14 involve changing the patterned embossing process toproduce modified articles that would then need to be finished in asecondary operation at the finishing station 42. The approach of FIGS.3-7 involves the positioning of relief gates, i.e., recesses, in eitherthe embossing pattern of primary embossing roller 30 or support roller32, which will result in a plurality of protrusions on the embossedarticle which must then be removed in the finishing station 42. Theapproach of FIGS. 8-14 is related, but is somewhat more complex in thatthe supporting roller 32 is machined to have a negative patterncorrelated with the positive embossing pattern of primary roller 30 sothat there is a substantially uniform cross-sectional area of theembossed part across the entire width of the sheet 12.

[0074] Referring now to FIG. 3, embossing station comprised of embossingrollers 30 and 32 is shown for embossing the sheet 12, and downstream ofthe embossing station is a finishing station 42 which is onlyschematically illustrated. The finishing station 42 will provide a meansfor grinding or slicing off unwanted material from the embossed articlesas further described below.

[0075] It will be understood that upstream of the embossing rollers 30and 32 will be conventional smooth cylindrical precalendering rollersand an impregnation, drying and calendaring station so that the sheet 12of material entering the embossing rollers 30 and 32 is a smooth sheetof flexible graphite material of uniform thickness, and of uniform resinconcentration across its width. In the processes of FIGS. 3-7 and thatof FIGS. 8-14, the material movement required for the formation ofcomplex shapes in the embossing process will be accommodated byproviding cavities in the rollers 30 and/or 32 which allow unwantedmaterial to flow into protrusions which will subsequently be machinedaway at the finishing station 42.

[0076] For example, FIG. 4 shows a plan view of an article 36 which isto be formed. The article 36 has a region 44 where the substantialmaterial flow is required in the embossing process.

[0077]FIG. 5 illustrates one means in which the flow from the area 44may be accommodated. In FIG. 5, there is schematically illustrated gatesor recesses 46 which will be formed in the outer surface of thesupporting roller 32 adjacent the periphery of the article 36 which willresult in protrusions extending from the periphery of the article 36after the sheet 12 moves through the embossing station 30, 32. Thoseprotrusions will be machined away in the finishing station 42.

[0078] Alternatively, FIGS. 6 and 7 illustrate the positioning of gates48 in the supporting roller 32 directly behind the area 44 so thatprotrusions will be formed on the backside of the article 36. Again,those protrusions will be machined away at finishing station 42.

[0079] In the process of FIGS. 3-7, the relief gates such as 46 or 48are positioned at high stress areas where material needs to be removedfor proper part definition and dimension. These gates 46 or 48 can bepositioned at the periphery of the part as shown in FIG. 5 or moredesirably at locations within the plane where high, localized regions ofmaterial flow or removal are needed as seen in FIGS. 6 and 7. The gatescan be located to promote material movement in-plane toward the outsideof the plate, as shown in FIG. 5, or to the bottom of the part, i.e.through-plane movement, as shown in FIGS. 6 and 7. The through-planemovement of FIGS. 6 and 7 is believed to be more advantageous since itwould allow gate positioning anywhere on the part 36 and not only aroundits perimeter.

[0080] Turning now to the process illustrated in FIGS. 8-14, this can begenerally described as a constant cross-section process which wouldyield substantially only through-plane resin and material flow for thearticles 36. As previously noted, it is desirable to have primarilythrough-plane flow since no material would then have to be pushed awaylaterally during the embossing operation. Achieving this substantiallyall through-plane flow would be possible through machining a negativepattern 52 on the bottom roller 32 to create excess volume for thematerial to move to during the embossing areas which will be relativelythin areas of the finished article.

[0081] Thus, as shown in FIG. 8, the primary embossing roller has anembossing pattern 50 defined thereon which includes raised areas whichwill result in corresponding thin areas of the embossed article. Thesupport roller 32 will have a complementary negative pattern 52 definedtherein to receive the material displaced by the positive embossingpattern 50. It will be understood that the positive and negativepatterns 50 and 52 are only schematically illustrated in FIG. 8, and noattempt has been made to illustrate the exact patterns which wouldresult in the article shown in FIGS. 9-14.

[0082] As shown in FIGS. 10 and 11, this will result in a fin or ridge54 which may be generally referred to as a protrusion 54 on the backsideof the embossed article 36. That protrusion 54 will then be removed inthe finishing station 42.

[0083] Both the method of FIGS. 3-7 and that of FIGS. 8-14 may bedescribed as methods for manufacturing articles 36 having a variablethickness from a sheet of flexible graphite material 12 of uniformthickness, which methods include the steps of:

[0084] (a) providing the sheet 12 of flexible graphite material having asubstantially uniform thickness across a width 38 of the sheet;

[0085] (b) embossing the sheet 12 between an embossing element 30 and alanding element or support roller 32 having embossing and landingsurfaces, respectively, at least one of which surfaces includes a firstrecess (46, 48 or 52) for forming a protrusion (such as 54) on theembossed sheet; and

[0086] (c) removing the protrusion thereby providing the article 36having a variable thickness 56 along a dimension 58 of the articlecorresponding to the width 38 of the sheet 12.

[0087] Thus, it is seen that the methods of the present inventionreadily achieve the ends and advantages mentioned as well as thoseinherent therein. While certain preferred embodiments of the inventionhave been illustrated and described for purposes of the presentdisclosure, numerous changes in the arrangement of steps may be made bythose skilled in the art, which changes are encompassed within the scopeand spirit of the present invention as defined by the appended claims.

What is claimed is:
 1. A process of forming articles from a graphitematerial, the process comprising steps of: (a) providing a sheet ofvariably impregnated flexible graphite material having a variable resinconcentration across a width of the sheet; and (b) passing the variablyimpregnated sheet past an embossing roller having an embossing patternof variable cross-section across the width of the sheet, the embossingpattern having a raised area which corresponds to an area of lesserresin content of the variably impregnated sheet, so that an amount ofresin which must be moved in the plane of the sheet during the embossingprocess is reduced as compared to a sheet having uniform resin contentacross its width.
 2. The process of claim 1, wherein step (a) furthercomprises: (a)(1) precalendering the sheet when the sheet is in the formof an unimpregnated mat, so that the mat has a variable density acrossthe width of the sheet; and (a)(2) impregnating the variable density matwith resin, so than an area of higher density mat becomes an area oflower weight % resin concentration across the width of the sheet.
 3. Theprocess of claim 2, wherein step (a)(2) further comprises applying aconstant resin solution across the width of the sheet, so that thevariable resin concentration across the width of the sheet is a functionsolely of the variable density of the mat of step (a)(1).
 4. The processof claim 2, further comprising: between step (a)(2) and step (b), dryingand calendering the resin impregnated mat.
 5. The process of claim 2,wherein: step (a)(1) further comprises creating at least three areaseach having a different mat density across the width of the sheet. 6.The process of claim 1, further comprising: severing the embossed sheetinto a plurality of articles, each article having a variable thicknessacross the width of the sheet.
 7. The process of claim 6, wherein thearticles are components for a fuel cell.
 8. The process of claim 6,wherein the articles are flow field plates for a fuel cell.
 9. A methodof manufacturing articles having a variable thickness from a sheet offlexible graphite material of uniform thickness, said method comprisingthe steps of: (a) providing the sheet of flexible graphite materialhaving a substantially uniform thickness across a width of the sheet;(b) embossing the sheet between an embossing element and a landingelement having embossing and landing surfaces, respectively, at leastone of which surfaces including a first recess for forming a protrusionon the embossed sheet; and (c) removing the protrusion thereby providingan article having a variable thickness along a dimension of the articlecorresponding to the width of the sheet.
 10. The method of claim 9,wherein: in step (b), the first recess is positioned to accommodatein-plane movement of material within the sheet during the embossingstep.
 11. The method of claim 10, wherein: the first recess ispositioned adjacent a periphery of the article.
 12. The method of claim10, wherein: in step (b), at least one of the surfaces includes a secondrecess positioned to accommodate through-plane movement of materialwithin the sheet during the embossing step.
 13. The method of claim 9,wherein: in step (b), the surfaces include a plurality of recessespositioned so that the embossed sheet has a substantially constantcross-section across the width of the sheet, whereby substantially allmaterial movement within the sheet during the embossing step isthrough-plane movement.
 14. The method of claim 9, wherein step (a)further comprises: (a)(1) impregnating the sheet with resin; (a)(2)drying the sheet; and (a)(3) calendering the sheet.
 15. The process ofclaim 9, wherein the article is formed into a component of a fuel cell.16. The process of claim 9, wherein the article is formed into a flowfield plate for a fuel cell.